This invention relates to systems having a high coefficient of performance (COP) in translating heat input to an intermediate output level suitable for such applications as central and residential heating. More particularly, this invention relates to heat pump systems, in which compression and expansion cycles of a compressible fluid are utilized to improve the COP by extracting heat from an ambient source.
The increased cost and lessened availability of traditional thermal energy sources (wood, coal and petroleum products) have caused investigations to be undertaken of a significant number of novel systems for achieving a higher coefficient of performance. There is a widespread need for intermediate level heating, by which is meant the range of temperatures usually employed for water, residential and central system heating. Conventional electrically driven heat pumps are employed for many air and water heating applications in which an intermediate heat level output is desired, because of the fact that a substantial energy contribution from ambient sources can be used. Although a significant improvement over conventional direct heating techniques, the COP based on power plant heat input still remains relatively low (e.g. of the order of 1.2), and when electrical generating and transmission losses are considered the COP is further reduced to values of 1.0 or less. Consequently, for some time prime movers have been used for direct driving vapor compression (Rankine) heat pump systems, and many installations currently employ diesel engines or Otto engines in these configurations. The net COP of these systems is still not desirably high, and substantial improvements in efficiency are highly unlikely for either the prime mover or the vapor compression (Rankine) heat pump device alone.
More recently, therefore, other workers in the art have considered the use of heat engines in conjunction with heat pumps for converting input energy into intermediate level heat. Examples of two such systems are provided in an article entitled "A Stirling Engine Heat Pump System" by M. L. Hermans and G. A. A. Asselman, published in the Proceedings of the Thirteenth Inter-Society Energy Conversion Engineering Conference, Volume 3, pp. 1830-1833 (1978). The laboratory systems described basically comprise an air-to-water Rankine heat pump driven by a Stirling heat engine and a somewhat modified system of the same type using a generator and a speed control arrangement. The COP for this laboratory system is stated to be in the range of 1.4 on a seasonal performance basis, with a maximum COP of approximately 1.5 derived at higher ambient temperature levels. The authors specifically point out, however, that redesign of the Stirling engine for this particular application is required, and that the working fluid sealing problem of the Stirling engine has still to be solved. Because central and residential heating systems are required to operate on a high reliability, long term basis with minimum maintenance expenditure, the Stirling engine does not appear at this stage to represent a viable alternative for intermediate temperature level output systems.
Other Stirling engine driven heat pump systems are described in the referenced article by Hermans et al. Further references are given in another article entitled "The Study Of The Gas Heat Pump System Driven By A Stirling Engine", by Y. Ishizaki et al, published in the Proceedings of the Fourteenth Inter-Society Energy Conversion Engineering Conference, pp. 2045-2049 (1979). This is a comparative study showing that the COP of the Stirling engine driven gas heat pump is higher than that of the Rankine and Otto cycles.
Without appearing to have considered intermediate level heating needs specifically, other workers have devoted attention to employment of the Vuilleumier cycle in heating and cooling systems. The Vuilleumier cycle, described first by Rudolph Vuilleumier in U.S. Pat. No. 1,275,507 (issued Aug. 13, 1918) and entitled "Method And Apparatus For Inducing Heat Changes" has certain significant advantages over the Stirling engine. As pointed out in the treatise "Stirling-Cycle Machines" by G. Walker, published by the Clarendon Press, Oxford University, 1973, at p. 134, the Vuilleumier machines "offer many alternative attractions on grounds of simplicity; lack of pistons and seals being the primary advantages". Machines utilizing the Vuilleumier cycle employ the cycling of various volume devices in predetermined phase relationships and with interchange of heat energy such that as work increases the hot end tends to get hotter and the cold end tends to get colder. Unlike the Stirling machine, mechanical energy is typically necessary to cycle the displacer elements, but because of the low differential pressures the amount of mechanical work that must be added to this thermodynamic system is not significant. As mentioned in the Vuilleumier patent the machine can be used for high temperature heating or for cooling, and in fact it has more recently been used in a number of miniaturized cryogenic refrigerator systems. What is referred to as a "duplex machine" comprising two Stirling engine mechanisms (pp. 108 and 109 of Walker) may be used as a "duplex gas-fired air-conditioning unit". In the Walker book, however, at page 134, the Vuilleumier machine is described as similar to the duplex Stirling-cycle engine, and the duplex machine on pp. 108 and 109 may in fact be regarded as of the Vuilleumier type.
A related disclosure is contained in an article entitled "Regenerative Gas Cycle Air Conditioning Using Solar Energy" by M. S. Crouthamel and B. Shelpuk, published by the National Technical Information Service as PB-270154 (Aug. 1975). This system is intended to function as a water cooler for air-conditioning applications, using a solar powered Vuilleumier cycle. The usage of solar energy to augment thermal output is a well understood expedient that has been widely considered in the scientific literature. Whatever the available thermal energy source, whether air, water, solar or ground, a heat pump system should be able to function with higher COP and preferably without the cost and complexity introduced by the use of separate systems, or the developmental problems inherent in machines such as the Stirling engine.
It is known in these Vuilleumier refrigerators to dump some thermal energy from the regenerator, as shown by U.S. Pat. No. 3,423,948, for the purpose of rejecting heat to ambient from the passing refrigerator fluid. This rejection is used in a minor amount to bias temperature changes in the cold direction, in the refrigerator type of application. As will be evident hereafter, however, the thermodynamic process must be viewed as a whole if useful output at intermediate temperature levels is to be derived with a COP in the range of 1.5 to 2.5. More specifically, the machine must be taken from the theoretical realm, in which the cycle may function in a fashion approaching the adiabatic, with low heat output, and placed in a practical context. In this context high specific heat output should be derived with high COP utilizing the thermal input derived from a fuel as well as the contribution from ambient sources to best advantage in a system which is inherently reliable.